The present invention relates to a pressure detecting device for detecting air pressure, which is preferably incorporated in a control system for a press machine and a stroke ends detecting system for a pneumatic cylinder or the like.
As well known, a pressure receiving member in the pressure detecting device employed for a pneumatic-electric signal converter or the like is constructed by a piston or plunger in the high pressure area or by a diaphragm in the low pressure area or by a Buldon tube for the both high and low pressure areas.
The conventional pressure detecting device with a piston incorporated therein is typically shown in FIG. 1. The detector housing 10 is formed with an axial bore 102 in which a piston 103 that is a pressure receiving member is axially displaceably inserted. On the outer surface of the piston 103 is arranged an annular seal 104 by means of which the pressure detecting port 105 and the discharge port 106 are air-tightly separated from each other. An actuating rod 107 is secured to the piston 103, which is displaceably extended outward through the side wall 101a at the discharge port side of the detector housing. Said actuating rod 107 is intended, for instance, for shifting a limit switch to ON position or OFF position by way of forward and backward movement thereof. Further the piston 103 is energized toward the pressure detecting port 105 by means of a coil spring 108. Now the resilient force of the coil spring 108 is defined as F. As pressurized air flows through the pressure detecting port 105 into the pressure chamber of the housing, the piston 103 is subjected to the leftward force PS.sub.o, as shown in FIG. 1, where P is air pressure and S.sub.o is area of the piston 103. Assuming that F.sub.o is resilient force of the coil spring 108, when the piston 103 comes in close contact with the side wall 101b of the housing, the following equation is applicable, as air pressure P reaches the predetermined pressure P.sub.o. EQU F.sub.o =P.sub.o S.sub.o ( 1)
As air pressure P is further increased, the piston 103 starts to be displaced against the resilient force F.sub.o of the coil spring 108 in the leftward direction in FIG. 1. When the piston 103 is displaced by distance l, the resilient force F of the coil spring 108 is defined by the following equation, EQU F=F.sub.o +Kl (2)
where K is spring coefficient. Since this resilient force F is equal to force caused by input air pressure, the following equation is applicable. EQU F=P S.sub.o ( 3)
Then, when putting Equations (1) and (2) into Equation (3), the following equation is obtained. ##EQU1## This equation can be represented by FIG. 2. Hence the operating characteristics of the pressure detecting device is such that as input air pressure P is increased, displacement distance l is linearly increased, as illustrated in FIG. 2. As input air pressure P reaches the predetermined pressure P.sub.s, the actuating rod 107 is displaced by distance l.sub.s. At this moment the limit switch (not shown) is shifted to ON position or OFF position so that the required pressure detecting is performed.
Since there is existent a linear relation between displacement distance l and input air pressure P with the conventional pressure detecting device, as illustrated in FIG. 2, the following disadvantages are pointed out with it.
Firstly, the pressure detecting device has a short durability of life particularly due to wearing of the seal, because the actuating rod is vibratively displaced forward and backward with the seal ring which is always in contact with the wall face, as air pressure fluctuates. If, however, the pressure detecting device is made less sensitive against pressure fluctuation, response accuracy is reduced.
Secondly, it is difficult to detect two pressures with high pressure ratio therebetween, because high pressure ratio requires long displacement distance and the conventional presure detecting device fails to meet this requirement due to the inherent structure thereof.
Moreover the seal member 104 which serves for providing air tightness between the pressure detecting port 105 and the discharge port 106 brings about the following disadvantages. Firstly, the piston 103 is irregularly displaced due to frictional force which is effective between the seal 104 and the inner wall face of the detector housing 101, causing the piston 103 to be stopped sometime. As a result response accuracy is reduced. Further the frictional force makes it impossible to carry out pressure detecting at the lower pressure area. Furthermore the pressure detecting device cannot be used for frequent operation, because the seal 104 wears off due to sliding contact against the inner wall face of the housing, as the piston 103 is displaced. As a result service life of the pressure detecting device is shortened.
It is to be noted that the drawbacks with the conventional pressure detecting device with a piston incorporated therein as mentioned above are common problems which are applicable to those with a plunger incorporated therein.
In the meantime, the pressure detecting device in which a diaphragm is arranged has also drawbacks that it is employed only at the low operation pressure area and that it has a short durability of life because of repeated deformation of the diaphragm operation. The pressure detecting device with a Buldon tube incorporated therein has also similar drawbacks of reduced response accuracy, short life or the like. These conventional pressure detecting devices in the diaphragm or Buldon tube type have linear characteristic in respect of relation of displacement distance l to input air pressure P, as illustrated in FIG. 2. Thus they cannot be free from the aforesaid drawbacks which are brought about by their structural condition.